Dielectric barrier discharge lamp

Abstract: (Object) The objects of the invention, in the case of a dielectric barrier discharge lamp, are first to obtain a good space uniformity of the light output, a good time stability of the light output as well as a high light yield, second to achieve a high light coefficient of utilization and at the same time a simple maintenance of the lamp and third not to contaminate a given atmosphere by air or the like. (Arrangement) In a dielectric barrier discharge lamp, in which a discharge vessel, in which a discharge gas forming "excimer" molecules is encapsulated, is provided with a dielectric provided with a netlike electrode and a window, through which the light goes, the first object of the invention is achieved by an arrangement in which a thickness of ends of the above-described netlike electrodes is greater than the average thickness of the entire electrode. The second object of the invention is achieved by an arrangement in which a holder incorporated in the discharge vessel has an outer dimension that is less than/equal to an outer dimension of the netlike electrodes. The third object of the invention is achieved by an arrangement by which the discharge vessel has a hollow cylindrical shape formed from an external tube and an internal tube and a means for hermetic sealing is arranged inside the internal tube.

Description: The invention relates to a so-called dielectric barrier discharge lamp, which is used, for example, as an ultraviolet light source for a photochemical reaction, and in which light radiated from "excimer" molecules, which are formed by a dielectric barrier discharge, is used. As generic art, a radiator, i.e., a dielectric barrier discharge lamp, is known, for example, from EP-A-324 953 (JP laid-open specification HEI 2-7353) or U.S. Patent US-A-9 837 484, in which a discharge vessel is filled with a discharge gas forming "excimer" molecules and in which "excimer" molecules are formed by a dielectric barrier discharge, which is also designated as ozone production discharge or as silent discharge, as is described in "Discharge Handbook," Electrogesellschaft [Electric Company], June 1989, 7th Edition, page 263. In the radiator, light is radiated from the "excimer" molecules. In the above-named publications, an arrangement of a dielectric barrier discharge lamp is described, in which the discharge vessel has a cylindrical shape and functions at least partially also as dielectric of the above-described dielectric barrier discharge, which is at least partially transparent relative to the light radiated from the above-described "excimer" molecules. In this discharge lamp, further, the above-described light-transmitting dielectric is provided at least partially with netlike electrodes. Further, another design of a dielectric barrier discharge lamp is known, which has an approximately cylindrical outer shape as well as an overall hollow cylindrical discharge vessel, in which an external tube and an internal tube are arranged coaxially to one another, a discharge space exists between the external tube and the internal tube and a hollow space is formed inside the internal tube. The above-described dielectric barrier discharge lamps have various advantages, which neither conventional low-pressure mercury discharge lamps nor conventional high-pressure arc discharge lamps have, such as, for example, a radiation of ultraviolet rays with short waves, in which main wavelengths lie at 172 nm, 222 nm and 308 nm, and at the same time a selective production of light with individual wavelengths with a high efficiency, which are, for example, line-spectrum-like. However, a conventional dielectric barrier discharge lamp had the drawback that a space uniformity of light output, a time stability and a light yield were not always obtained to a sufficient degree. Further, it was regarded in this connection as disadvantageous that despite the lamp arrangement that is completely different from the conventional low-pressure mercury discharge lamp or the conventional high-pressure arc discharge lamp, no adequate examination of a coefficient of utilization of the light or of a maintenance of the lamp was performed. Such a dielectric barrier discharge lamp is used for reforming plastic surfaces, for forming layers or for similar purposes. The above-described drawbacks are characteristic for a dielectric barrier discharge lamp, which, for example, has a hollow cyclindrical discharge space, which is designed so that an external tube as well as an internal tube with approximately cylindrical outer shapes are arranged coaxially to one another. These drawbacks occur especially when using the dielectric barrier discharge lamp for the purpose of a photochemical reaction. An object of the invention is therefore to indicate a dielectric barrier discharge lamp, which has at its disposal an advantageous (good) space uniformity of the light output, an advantageous time stability and at the same time a high light yield. The object is achieved according to the invention in that in a dielectric barrier discharge lamp, in which a discharge vessel is filled with a discharge gas forming "excimer" molecules by a dielectric barrier discharge, in which the above-described discharge vessel functions at least partially also as dielectric of the above-described dielectric barrier discharge and is at least partially transparent relative to the light radiated from the above-described "excimer" molecules, and in which the above-described dielectric is provided at least partially with electrodes, a means is arranged by which a thickness of electrode ends of the above-described electrodes is greater than the average thickness of the above-described electrodes. The object of the invention is further advantageously achieved in that for the above-described means, the above-described electrode ends are wrapped with a wire, a twisted wire, a metal strip, and/or a strip made of metal netting. The object of the invention is also advantageously achieved in that ends of seamless, cylindrical, netlike electrodes, which have a resilience in axial direction of the above-described discharge lamp, are folded. Moreover, the object of the invention is advantageously achieved in that an electrode lead is connected to the wire, the twisted wire, the metal strip and/or the strip made of metal netting, with which the above-described electrode ends are wrapped, or in that a conductive paste is applied to the ends of the above-described electrodes. With respect to the object of the invention, the inventors have discovered the following: A dielectric barrier discharge lamp consists of a multiplicity of microscopically small discharge plasmas with a very small plasma diameter and at the same time a very short discharge period, which are designated below as microplasmas, as is described in the above-named "Discharge Handbook." In the dielectric barrier discharge lamp, a stability of the light output, a space uniformity as well as a light yield are influenced by electrode ends incorporated in a dielectric. These objects can be achieved by an arrangement in which a thickness of the electrode ends is greater than the average thickness of the above-described electrodes. The reaction process for the improvement of the above-described time stability of the light output, space uniformity as well as the light yield is not quite completely explained, but it functions presumably as follows: Electrodes are basically thin and tend to have a nonuniform and great field strength on their ends, since the ends are often shaped knife-edge-like or needle-like. A creep-discharge-like discharge as well as a multiplicity of microplasmas develop in an intensive way on the ends of the electrodes, therefore not in a discharge gas but in an atmospheric gas, such as air or the like. As a result, the discharges become unstable, the light output on the electrode ends is great, i.e., a time fluctuation of the light output is great and the space uniformity of the light output deteriorates. Further, the light yield also drops, if a creep-discharge-like discharge or an excessively intensive production of microplasmas develops in an atmospheric gas, such as air or the like. By the arrangement for achieving the first object of the invention, the intensification of the field strength on the electrode ends is reduced, and becomes relatively uniform and weak. As a result, the space uniformity of the light output, the time fluctuation of the light output as well as the light yield are improved. Further, the thickness of the electrode ends can be increased in a simple and at the same time uniform way by the measure by which the electrode ends are wrapped with a wire, twisted wire, metal strip, or strip made of metal netting or several of them, or by which the above-described electrode ends are folded. Moreover, a dielectric barrier discharge lamp with high reliability can be obtained by the measure according to the invention in which an electrode lead is connected to the wire or the like, with which the above-described electrode ends are wrapped, since the connection of the above-described electrodes is performed with high mechanical strength and reliability. According to the invention, no unnecessary discharge results even by the arrangement by which as electrodes, seamless, cylindrical, netlike electrodes are arranged, which have a resilience in axial direction of the lamp, since the radius of the above-described netlike electrodes is reduced, comes to lie with the dielectric head to head, adjoining one another completely tightly, and thus no hollow space is formed in a part if the netlike electrodes are pulled in axial direction, after the discharge vessel was inserted in them. As a result, a production of harmful compounds in the area surrounding the lamp is prevented because of this unnecessary discharge and at the same time a stable discharge and thus a stable light output are obtained and the light yield is increased. This means that a dielectric barrier discharge lamp can be obtained, which has a space-uniform discharge, a stable discharge as well as a stable light output, since the above-described netlike electrodes on a surface of the approximately cylindrical dielectric have a sufficient uniformity, without an overlapping resulting, as in the formation of a suture line by bunching of the netlike electrodes. According to the invention, furthermore, by the measure in which a conductive paste is applied to the electrode ends, whose main component is silver, gold, nickel, carbon or the like, the thickness of the electrode ends can be increased in a simple way and with any shape and thus the uniformity of the field strength can be largely improved. If the conductive paste is applied to the component with which the electrode ends are wrapped, such as wire or the like, a dielectric barrier discharge lamp with a high reliability can be obtained, since the connection of the above-described electrodes is performed with an even higher mechanical strength and reliability. Below, the invention is further described based on the embodiments represented in the drawing. There are shown in: Fig. 1 a diagrammatic representation of an embodiment of the dielectric barrier discharge lamp according to the invention; Fig. 2 a diagrammatic representation of netlike electrodes of the dielectric barrier discharge lamp according to the invention; Fig. 3 a diagrammatic representation of another embodiment of the dielectric barrier discharge lamp according to the invention; and Fig. 4 a diagrammatic representation of still another embodiment of the dielectric barrier discharge lamp according to the invention. First, embodiments for achieving the object of the invention are shown in Fig. 1: In the representation, a reference symbol 1 designates a discharge vessel, which is shaped like a hollow cylinder so that an internal tube 2 made of synthetic quartz glass and an external tube 3 made of synthetic quartz glass are arranged coaxially to one another. Discharge vessel 1 actually has, for example, a total length of about 150 mm, an outer diameter of the internal tube of 14 mm, an inner diameter of the external tube of about 25 mm as well as a thickness of 1 mm. On its outer surface, internal tube 2 has an aluminum electrode 5, which also functions as a light-reflector disk. A barium getter 6 is arranged on one end of discharge vessel 1. Electrode 5 is formed by cathode sputtering and has a thickness of, for example, 0.005 mm. External tube 3 also functions as a dielectric of a dielectric barrier discharge as well as as a light exit window and has a netlike electrode 4 on its outer surface. Netlike electrode 4, as partially illustrated in Fig. 2, is designed so that a metal wire 23 is made knitted seamless and cylindrical and loops are repeatedly made in peripheral direction 22a-22b of the cylinder. Metal wire 23 consists, for example, of monel with a litz wire diameter of 0.1 mm. A large mesh 24 and a small mesh 25 have an area of about 2 mm 2 and an area of about 1 mm 2 , respectively. Netlike electrode 4, which is arranged head to head tightly adjoining one another on an outer side of external tube 3, is designed so that discharge vessel 1 is inserted in this cylindrical metal netting and is pulled in axial direction of the lamp. By this arrangement, netlike electrode 4 is arranged on external tube 3 head to head tightly adjoining one another. The cylindrical netlike electrode has an outer diameter of, for example, about 27.4 mm. For electrode 4, a conductive, netlike electrode is thus used in a suitable way. But it is also possible to design electrode 4 from a light-transmitting thin layer. Xenon gas is encapsulated as discharge gas with a pressure of, for example, 40 kPa (300 torrs) in a discharge space 7 of discharge vessel 1. For example, in a discharge with an output of 2 W/cm lamp, by using a source of current 8 with a frequency of 20 kHz, ultraviolet rays with a wavelength of 172 nm and in the range of this wavelength were radiated with high efficiency. A gap of discharge space 7 lies, for example, at 5.5 mm. Ends 11a and 11b of netlike electrode 4 were wrapped with rust-free wires 12a and 12b with a diameter of, for example, about 0.1 mm in axial direction of the lamp over a length of, for example, about 3 mm. An electrode lead 19 was connected to rust-free wire 12a, to which a silver paste 13 is applied in a thickness of about 0.5 mm. Silver pastes 14a and 14b, for example, were applied in a thickness of about 0.5 mm and in a length of 5 mm to ends 10a, 10b of interior electrode 5. A uniformity of the field strength in axial direction of the lamp can be achieved by the above-described measure, in which the rust-free wires are added to the electrode ends and thus the thickness only of the ends of the electrodes is increased and an average thickness of the electrode ends is greater than the average thickness of the entire electrode. A dielectric barrier discharge lamp can therefore be obtained, in which the space uniformity of the light output, the time fluctuation of the light output as well as the light yield are improved. The term "electrode end" is to be understood to mean a part of an electrode, which extends from an end part of the electrode, i.e., from a part in which the electrode comes to an end along the dielectric of the dielectric barrier discharge, in a length which is less than/equal to a length of the discharge gap of the dielectric barrier discharge. As means which is added to increase the thickness of the electrode ends, wire, twisted wire, metal strips and/or strips made of metal netting is/are used. In Fig. 3, another means is shown, by which the average thickness of the electrode ends becomes greater than the average thickness of the entire electrode. In this case, netlike electrode 4 was bent on its ends 11a and 11b, so that they come to lie on one another and folds 30a and 30b are formed, to which nickel pastes 31a and 31b, for example, were applied in a thickness of about 1 mm. By bending or folding the electrodes on their ends, an average thickness of the electrode ends greater than the average thickness of the entire electrodes can be achieved. As a result, an improvement of the space uniformity of the light output, the time fluctuation of the light output as well as the light yield can be achieved. Fig. 4 illustrates another embodiment. In this embodiment, no hollow-cylindrical arrangement is shown, in which an internal tube and an external tube are arranged coaxially to one another, as in the above-described embodiment. A disklike discharge vessel 49 is made from a platelike dielectric 41 made of synthetic quartz glass, a platelike discharge vessel component 45 made of aluminum as well as cylindrical quartz glass 46. Platelike dielectric 41 has, for example, a diameter of 100 mm and a thickness of 1.5 mm. Dielectric 41 functions both as dielectric for a dielectric barrier discharge and as light exit window component and has on its outer surface a metallic, netlike electrode 43 with a diameter of 80 mm, which is designed so that rust-free litz wires with a diameter of about 0.1 mm are knitted at a distance of, for example, 2 mm by orthogonal crossing. Inner electrodes consist of an electrode 44a made of an aluminum rod, which is incorporated in the center of above-described vessel component 45, as well as of electrodes 44b and 44c made of aluminum rings in the area surrounding electrode 44a. The electrodes each have a diameter of, for example, 1 mm. Electrodes 44a, 44b and 44c also function as reinforcing component against an atmospheric pressure of dielectric 41. Electrodes 44a, 44b and 44c adjoin dielectric 41, but are not mechanically fastened in dielectric 41. Parts of electrodes 44a, 44b, 44c, as well as of vessel component 45, opposite the discharge space, are covered with a dielectric 40 consisting of MgF 2 . A rust-free ring 50, for example, with an inner diameter of 75 mm, an outer dimension of 85 mm and a height of 2 mm was arranged on an electrode end 48 of netlike electrode 43 by an electric contact with netlike electrode 43. Ring 50 was fastened by an adhesive based on silicone rubber 51 to dielectric 41. As discharge gas, xenon gas was encapsulated with, for example, 350 torrs in a hollow space 47, and an alternating voltage was applied by source of current 8 between electrodes 43, 44a, 44b and 44c. In this connection, a creep discharge plasma 52 was produced near dielectric 41 and ultraviolet rays were radiated with a high efficiency from "excimer" molecules of xenon, which have a maximum value at a wavelength of 172 nm and in the range of this wavelength. Also in this case, an average thickness of the electrode ends greater than average thickness of the entire electrode is achieved by an addition of rust-free ring 50 in electrode end 48. An improvement of the space uniformity of the light output, the time fluctuation of the light output as well as the light yield can therefore be achieved and thus a dielectric barrier discharge lamp with a smaller shape in comparison to the above-described embodiment can be obtained. As described above, a good space uniformity of the light output as well as a good time stability of the light output and at the same time a high light yield can be obtained by the dielectric barrier discharge lamp according to the invention.

Claims: Dielectric barrier discharge lamp, in which a discharge vessel (1, 49) is filled with a discharge gas forming "excimer" molecules by a dielectric barrier discharge, in which the discharge vessel functions at least partially also as a dielectric (3, 41) of the dielectric barrier discharge and is at least partially transparent relative to the light radiated from the "excimer" molecules, and in which the dielectric is provided at least partially with electrodes (4, 5, 43, 44a-c), characterized in that a means is arranged, by which an average thickness of electrode ends (10a, 10b, 11a, 11b, 48) of the electrodes is greater than the average thickness of the electrodes. Dielectric barrier discharge lamp according to claim 1, wherein for the means, the electrodes ends are wrapped with a material (12a, 12b), in which a wire, a twisted wire, a metal strip and/or a strip made of metal netting is/are selected. Dielectric barrier discharge lamp according to claim 2, wherein an electrode lead (19) is connected to the wire (12a), the twisted wire, the metal strip, and/or the strips made of metal netting, with which the electrode ends (11a, 11b) are wrapped. Dielectric barrier discharge lamp according to claims 1 to 3, wherein the means is formed by folding (30a, 30b) the electrodes (4) on their ends (11a, 11b). Dielectric barrier discharge lamp according to claims 1 to 4, wherein the electrodes consist of a seamless, cylindrical, conductive netting (4), which has resilience in the axial direction of the discharge lamp. Dielectric barrier discharge lamp according to claims 1 to 5, wherein a conductive paste (13, 14a, 14b, 31a, 31b) is applied to the ends (10a, 10b) of the electrodes (4, 5). Dielectric barrier discharge lamp according to any one of the preceding claims, in which the discharge vessel (1) has a hollow cylindrical discharge space (7), which is designed so that an external tube (3) as well as an internal tube (2) with approximately cylindrical outer shapes are arranged coaxially to one another and in which the outer wall of the external tube (3) is at least partially transparent relative to the light radiated from the "excimer" molecules and at the same time also functions as a dielectric of the dielectric barrier discharge. Dielectric barrier discharge lamp according to claim 1, in which a disklike discharge vessel (49) is filled with a discharge gas forming "excimer" molecules by a dielectric barrier discharge, which discharge vessel (49) comprises a platelike dielectric (41) functioning also as light exit window component and being at least partially provided with electrodes (43), and which further comprises a platelike discharge vessel component (45) incorporating electrodes (44a, 44b, 44c) adjoining the dielectric (41).

Inventors: ASAHINA TAKASHI, HIRAMOTO TATSUMI, HISHINUMA NOBUYUKI, IGARASHI TATSUSHI, KASAGI KUNIO, MATSUNO HIROMITSU, OONISHI YASUO, TAKEMOTO FUMITOSHI, WAKAHATA YASUHIKO, ASAHINA, TAKASHI, HIRAMOTO, TATSUMI, HISHINUMA, NOBUYUKI, IGARASHI, TATSUSHI, KASAGI, KUNIO, MATSUNO, HIROMITSU, OONISHI, YASUO, TAKEMOTO, FUMITOSHI, WAKAHATA, YASUHIKO
Applicants: USHIO ELECTRIC INC, USHIODENKI KABUSHIKI KAISHA
Application year: 1994
Main classification: H01J65
IPC codes: H01J65